btrfs-progs: tests: 006-image-on-missing-device: fix btrfs tool path

btrfs-progs: tests: 006-image-on-missing-device: fix btrfs tool path

If btrfs isn't in the path, this test will fail with:

    [TEST/misc]   006-image-on-missing-device
failed: btrfs fi show /dev/loop0
test failed for case 006-image-on-missing-device
Makefile:226: recipe for target 'test-misc' failed
make: *** [test-misc] Error 1

Fix the test script by adding $TOP to the path.

Signed-off-by: Luis Henriques <henrix@camandro.org>
[ updated to full command names ]
Signed-off-by: David Sterba <dsterba@suse.com>